Featured Commentary: More, Better Research Needed on Marijuana’s Effects on Teens and Young Adults


A recent retrospective review published in JAMA Psychiatry demonstrates how little we know about the effects of marijuana use, particularly on teenagers and young adults. Researchers at the University of Pennsylvania Perelman School of Medicine compiled a meta-analysis of 69 studies conducted over the past decades and compared the cognitive performance of 2,152 cannabis users with that of 6,575 non-users.

The study is bad news for those claiming marijuana use has minimal effects; the review found a consistent negative effect across all studies. The potential impact of regular marijuana use on school performance and overall mental and emotional growth among teenagers and young adults remains worrisome.

Most of the studies required a period of abstinence before the tests, but very few took active steps to ensure that participants had abstained. In the 15 studies where a 72-hour period of abstinence was enforced, analysis found that there was no significant difference among the 928 participants in those studies between those who used marijuana and those who did not. The researchers at Penn took that to mean that the cognitive effects of marijuana quickly wear off and that concern over long-term cognitive impairment might be overblown. We are not so sure.

While this sort of retrospective analysis of results of older studies is a well-respected approach to medical research, the key is to screen the methodology of the underlying studies to ensure that the analysis is comparing apples to apples.

For example, the definition of a marijuana user varies wildly among the 69 studies. Someone who uses marijuana every weekend can certainly be called a “regular” marijuana user, but there may be a quantitative and qualitative difference from someone who uses every day. If we restricted to daily use, for example, we might expect much larger effect sizes.

The THC content in marijuana has gradually increased over the past two decades, as expressed as an average of the marijuana impounded by the police. Are the results of studies conducted 20 years ago still valid, or does the THC content of the marijuana have a bearing on long-term cognitive effects?

This is an important study, adding to our understanding that marijuana hurts cognition. However, we need to continue to improve our research methods to get a better understanding of the precise cognitive effects of today’s highly potent marijuana across different populations.

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